1. Field
This invention relates generally to the decommissioning of spent boiling water reactor control rods and more particularly to the removal of tritium from those control rods.
2. Related Art
One type of commonly used boiling water nuclear reactor employs a nuclear fuel assembly comprised of fuel rods surrounded by a fuel channel. Each fuel channel of a boiling water reactor fuel assembly typically includes a hollow, linear, elongated, four-sided channel of integral construction, which except for its rounded corner edges, has a substantially square cross section. Commonly, each channel is roughly 14 feet (4.27 meters) long by 5 inches (12.7 cms.) and laterally encloses a plurality of elongated fuel elements. The fuel elements are arranged to allow for the insertion of a cruciform-shaped control rod, which during reactor operation, is movable vertically to control the nuclear reaction. The control rods typically include an upper portion having a handle and four upper rollers for guiding the control rod as it moves vertically and a lower portion comprising a lower casting and lower ball rollers. The main body structure includes four blades or panels which extend radially from a central spline. Preferably, the blades extend longitudinally to a height that substantially equals the height of the fuel elements, which is approximately 12 feet (2.6 meters). The width of the control rods at the blade section is approximately twice the width of the panels, which is in the order of 10 inches (25.4 cms.).
Following functional service, boiling water reactor control rods are difficult to store and dispose of because of their size, configuration, embrittled condition and radiological activity. Heretofore, within the United States, in-pool storage of certain irradiated hardware has been extremely space inefficient and dry cask storage is not currently readily available. Accordingly, boiling water reactor operators must necessarily dispose of irradiated control rods as soon as reasonably practical.
Irradiated control rods are typically class C, low level radioactive waste as defined and determined pursuant to 10 CFR §61 and related regulatory guidance, e.g., NRC's Branch Technical Position on Concentration Averaging and Encapsulation. Since Jul. 1, 2008, low level radioactive waste generators within the United States that are located outside the Atlantic Compact (Connecticut, New Jersey and South Carolina) have not had access to class B or class C, low level radioactive waste disposal capacity. Lack of disposal capacity has caused boiling water reactor operators considerable spent fuel pool overcrowding. Though currently very uncertain and subject to numerous regulatory and commercial challenges, class B and class C low level radioactive waste disposal capacity for the remainder of the United States low level radioactive waste generators is anticipated in the relatively near future. Even when waste disposal sites become available, much of the irradiated control rods will be difficult and expensive to ship because of their size and configuration unless their volume can be significantly reduced and tightly compacted into licensed shipping casks. Disposal and long term and/or indefinite storage of control rod blades is technically challenging and commercially expensive. Decommissioning is typically best achieved by segmentation into predetermined sizes to achieve optimal physical and radiological characteristics. Segmentation is performed within the reactor facilities' spent fuel pool. One method of segmentation is described in co-pending Application Ser. No. 13/612,905, filed concurrently herewith, entitled METHOD OF SEGMENTING IRRADIATED BOILING WATER REACTOR CONTROL ROD BLADES. Generally, that method makes two orthogonal cuts longitudinally down the central spline of the control rod separating the cruciform formed blades into four equal panels, which can then be laterally segmented to a desired size suitable for cask storage or shipping.
More particularly, the principal components of one form of boiling water reactor control rod are the lifting handle, stellite roller bearings, velocity limiter, and the cruciform-shaped main blade body. The former components are positioned at the extremities of the control rod cruciform-shaped main body, and preferably, are removed in a manner consistent with the prior art as part of the control rod volume reduction process. The cruciform-shaped main body is comprised of four sheathed, metallic panels of metallic tubes containing powdered boron carbide or other neutron absorbing material, that are welded to a central spline lengthwise at opposing angles to fashion the cruciform shape. Underwater lateral segmentation of the panels ruptures both the sheathing and the tubes contained within the sheathing thereby exposing a spent fuel pool to unwanted debris in the form of sheathing materials, tubes and boron carbide. Embrittlement of the control rods caused by neutron exposure compounds the difficulty of lateral segmentation. The process described in the foregoing co-pending application solves this problem by wrapping a band of malleable metal laterally around the blades at the point where the cladding is to be laterally cut and crimping the malleable metal which seals the cladding at the point of segmentation so it can be cut.
While being serviced within the reactor pressure vessel, certain constituents of the control rod blades when exposed to the neutron fuel associated with nuclear fission cause the formation of the radioisotope tritium. Following useful service, continuing through storage and ultimately disposition, certain amounts of tritium may remain with the control rod blades that can be released into the spent fuel pool during the lateral segmentation process, which would be undesirable.
Accordingly, it is an object of this invention to remove the tritium from the control rods prior to lateral segmentation.
Furthermore, it is an additional object of this invention to capture the tritium removed from the control rod, in a stable form in which it can be easily contained.